What makes the heater not blow hot air.

If you verify the engine is getting hot enough, and the heater core has hot coolant going in and coming out, then you likely have a temperature blend door motor failed inside the truck. It is located behind the dash on the heater plenum, and it regulates the temperature of the air coming from the dash. When the actuator fails, it will no longer adjust the door...and so when you call for heat -- you still get cold air. Good luck.

heater coil